The light-emitting diode (LED) is a solid state semiconductor device, which has been broadly used as a light-emitting device. The light-emitting device structure comprises a semiconductor stack comprising a p-type semiconductor layer, an n-type semiconductor layer, and a light-emitting layer formed between the p-type semiconductor layer and the n-type semiconductor layer. The structure of the light-emitting device generally comprises III-V group compound semiconductor such as gallium phosphide (GaP), gallium arsenide (GaAs), or gallium nitride (GaN). The light-emitting principle of the LED is the transformation of electrical energy to optical energy by applying electrical current to the p-n junction to generate electrons and holes. Then, the LED emits light when the electrons and the holes combine.
For the light-emitting device, sapphire is often used as the growth substrate, but it is difficult to reduce the cost because the cost of the sapphire substrate is high.
FIG. 1 illustrates a cross-sectional diagram of a light-emitting device using a silicon substrate 10 as a growth substrate. A semiconductor layer 11 such as gallium nitride (GaN) is formed on the silicon substrate 10. The coefficient of thermal expansion (CTE) of the silicon substrate 10 is 3.59×10−6 K−1 and the CTE of the semiconductor layer 11 such as gallium nitride (GaN) is 5.59×10−6 K−1. The difference of the coefficient of thermal expansion (CTE) between the silicon substrate 10 and the semiconductor layer 11 is over 50%. The difference of CTE between the semiconductor layer 11 and the silicon substrate 10 provides a tensile stress as illustrated in FIG. 1. FIG. 2 illustrates a plane view of a surface of the light-emitting device. As shown in FIG. 2, the chipping and cracking of the light-emitting device easily occur accompanied with the tensile stress.